In testing for analytes such as drug molecules, immunoassays have proven to be especially useful. In an immunoassay, the interaction of an analyte, sometimes referred to as an antigen, with a specific receptor, typically an antibody, results in the formation of an antigen-antibody complex. This complex can be detected by various measurements, such as radioactivity, fluorescence, light absorption and light scattering. The results are then correlated with the presence, absence, and ideally the concentration of the analyte.
One type of particle-based agglutination immunoassay is based on the binding of an antigen with a particle-bound antibody. The particles employed are generally polymer particles, such as polystyrene and poly(methyl methacrylate), and are typically produced by an emulsion polymerization process. Other particle systems may also be used, including gold particles such as gold nanoparticles and gold colloids; and ceramic particles, such as silica, glass, and metal oxide particles. The antibody may be physically adsorbed onto the particle; however, greater stability and longer shelf-life are obtained when the antibody is covalently attached. See for example J. L. Ortega-Vinuesa et al. J. Biomater. Sci. Polymer Edn., 12(4), 379-408 (2001).
Particles having covalently bound antibodies are typically prepared by activation of the particles, followed by coupling of antibodies to the activated particles. For particles having carboxylate groups bound to the surface, activation is often achieved by contacting the particles with a solution of a carbodiimide coupling reagent and a succinimide reagent such as N-hydroxysuccinimide (NHS) or N-hydroxysulfosuccinimide (sNHS). The carboxylate groups on the surface are thus converted into NHS-ester or sNHS-ester groups. Carbodiimide couplers include, for example, N-ethyl-N′-(3-dimethyl-aminopropyl)carbodiimide (EDC); dicyclohexylcarbodiimide (DCC); and diisopropylcarbodiimide (DIC). Antibodies, for example IgG, can then be coupled to the particles by mixing the activated particles and the antibodies in an aqueous mixture, thereby forming sensitized particles. An illustration of this process is given in the following reaction scheme. 
The sensitized particles typically are then treated with a post-blocker, for example bovine serum albumin (BSA). In these processes covalent linkages, for example amide linkages (—NH—C(═O)—), are formed between the particle surface and the antibody and between the particle surface and the BSA. These covalent linkage formations are not typically exhaustive, and residual NHS-esters or sNHS-esters can remain on the particle surface.
When sensitized particles are mixed in an aqueous environment with a sample to be analyzed, the antigen in the sample will specifically bind to the antibody, thus causing the particles to agglutinate into clusters of particles having a larger collective size than that of an individual particle. This agglutination can be detected by measuring the change in the absorbance or the scattering of light by the sample. Ideally, the degree of agglutination in a particle-based agglutination immunoassay can be correlated with the amount of antigen in the sample. However, non-specific interactions between the particles and the sample can result in agglutination of the particles which is unrelated to the antigen-antibody interaction. These unwanted interactions can cause false positive or false negative results, and can also lead to inaccurate correlations of the agglutination response to the concentration of the antigen of interest.
It is believed that residual NHS-esters or sNHS-esters on particle surfaces can undergo non-specific chemical reactions with sample components, with polypeptide sample components being especially problematic. In order to minimize non-specific binding, sensitized particles have conventionally been treated with an amine compound such as glycine or ethanolamine to react with these esters before the particles are used in immunoassays (U.S. Pat. No. 5,486,479). These efforts have met with mixed success.
It is thus desirable to prevent chemical reactions between particle-bound NHS-esters or sNHS-esters and sample components. It is also desirable to provide a modified particle surface which inhibits adsorption of sample components onto the surface.